It is difficult to specify whether a fault factor occurred in transmission lanes is a module in a transmitting side or a module in a receiving side, when the fault occurs in the transmission lanes connecting the two modules in the lock-step fault tolerant apparatus. In specifying the factor of the fault in a fault tolerant apparatus, a CPU subsystem or an I/O subsystem in the two modules of the fault tolerant apparatus is functionally separated from the fault tolerant apparatus in accordance with a predetermined condition. CPU is an abbreviation of “Central Processing Unit”. I/O is an abbreviation of “Input/Output”. A subsystem as a fault factor in transmission lanes are functionally separated and a module including the separated subsystem is replaced. As a result, the replaced module and another module can recover duplicate state in the fault tolerant apparatus.
PTL 1 discloses an example of a fault tolerant apparatus.
The fault tolerant apparatus disclosed in PTL 1 includes three system buses and six processors. Each system bus is connected with two processors. In the fault tolerant apparatus, three-multiplexing state, at most, is realized by using processors connecting with each other via the system buses.